This application is a 371 of PCT/JP00/04253, filed Jun. 28, 2000, now WO 02/00383 A1.
The present invention relates to improvements in a method of wire electric discharge machining by which an electric discharge is generated in a gap between a wire electrode and a workpiece so that the workpiece is machined.
Electric discharge machining has been steadily used for machining metallic molds and others, that is, electric discharge machining has been widely used in the field of machining metallic molds in the automobile industry, electric appliance industry, semiconductor industry and so forth.
FIG. 8 is a schematic illustration for explaining a mechanism of electric discharge machining. In the drawing, reference numeral 1 is an electrode, reference numeral 2 is a workpiece, reference numeral 3 is an arc column, reference numeral 4 is a dielectric fluid, and reference numeral 5 represents chips created in the process of electric discharge machining. When the cycle including process (a) to process (e), which correspond to (a) to (e) shown in FIG. 8, is being repeated, electric discharge machining is conducted on the workpiece 2. That is, each process is conducted as follows.
(a) The arc column 3 is formed by the generation of electric discharge.
(b) The workpiece is partially melted by thermal energy of electric discharge and the dielectric fluid 4 is vaporized.
(c) A vaporizing explosion force of the dielectric fluid 4 is generated.
(d) A melting section (chips 5) is scattered,
(e) Cooling, coagulation and recovery of insulation between the electrodes are executed by the dielectric fluid 4.
The present invention relates to wire electric discharge machining used for gouging or cutting. In the field of wire electric discharge machining, there is a strong demand of enhancing the dimensional accuracy. For example, when a metallic mold used in the field of semiconductor industry is machined, it is necessary to conduct machining with high accuracy of 1 to 2 xcexcm.
FIG. 9 is a schematic illustration showing an example of the machining process of wire electric discharge machining. In the drawing, reference numeral 1a is a wire electrode, reference numeral 2 is a workpiece, reference numeral 4a is water which is used as a dielectric fluid, and reference numeral 6 is an initial hole. In FIG. 9, (a) is a view showing a circumstance of the first cut process in which rough machining is conducted, (b) is a view showing a circumstance of the second cut process in which intermediate machining is conducted after the rough machining, and (c) is a view showing a circumstance of the third cut process in which final machining is conducted.
In the example of the first cut process shown in (a) of FIG. 9, the wire electrode 1a is made to pass through the initial hole 6, and the workpiece 2 is gouged. In this case of the first cut process, the surface roughness and dimensional accuracy are finally finished in the process conducted later. Therefore, it is unnecessary to machine the workpiece with such a severe surface roughness and high dimensional accuracy in the process of first cut, but it is important to increase a rate of machining so as to enhance the productivity. In order to increase the rate of machining of wire electric discharge machining, water 4a is strongly sprayed to between the electrodes so that chips can be effectively ejected from between the electrodes. In order to uniformly spray water to between the electrodes and in order to prevent the breaking of wire of the wire electrode 1a, a method is used in which water 4a is stored in a processing tank not shown and the workpiece 2 is dipped in the processing tank.
According to the above conventional method of wire electric discharge machining, the second cut ((b) in FIG. 9) and the third cut ((c) in FIG. 9), which are conducted after the first cut ((a) in FIG. 9), are also conducted in water 4a which is a dielectric fluid.
FIG. 10 is a graph showing an example of the wave-form of voltage and the wave-form of a current between the electrodes. In the graph, V is a voltage between the electrodes, I is a current between the electrodes, and t is time. In the state at time Ti shown in FIG. 10, voltage is impressed upon between the wire electrode 1a and the workpiece 2. When voltage is impressed between the electrodes, the positive polarity and the negative polarity attract each other by a force of static electricity. By this force of static electricity, the wire electrode 1a, the rigidity of which is low, is drawn onto the side of the workpiece 2. This could be a cause of vibration of the wire electrode 1a. Due to the vibration, it becomes difficult to conduct wire electric discharge machining with high accuracy. In the conventional method of wire electric discharge machining, the above problems may be encountered.
In the state of time T2 shown in FIG. 10, a vaporizing explosion force of the dielectric fluid is generated by the energy of electric discharge, that is, for example, the state of time T2 is the same as that of (c) of FIG. 8. A high intensity of force, the direction of which is opposite to the workpiece 2, acts on the wire electrode 1a by the vaporizing explosion force of the dielectric fluid. By this high intensity of force, vibration is generated in the wire electrode 1a. When the above vibration is generated, irregularities are caused in the profile of the workpiece 2, and the dimensional accuracy is deteriorated.
In the field of semiconductor industry in which wire electric discharge machining is utilized, for example, in the process of machining a metallic mold of an IC lead frame, a very high accuracy of a profile not more than 1 xcexcm and very smooth surface roughness not more than 1 xcexcmRmax are demanded to the workpiece, and further it is necessary to enhance productivity. Especially when wire electric discharge machining is used for the above object, the above problems caused by vibration of the wire electrode are remarkable.
The present invention has been accomplished to solve the above problems. It is an object of the present invention to provide a method of wire electric discharge machining, the productivity of which is high, which is suitable for highly accurate machining.
The present invention provides a method of wire electric discharge machining by which an electric discharge is generated in a gap between a wire electrode and a workpiece so that the workpiece is machined, comprising the step of machining the workpiece while at least two machining processes in the machining processes of machining in a dielectric fluid, machining in mist and machining in gas are combined with each other.
Also, the present invention provides a method of wire electric discharge machining, in which the machining process is changed over between said machining processes when straightness of the workpiece reaches a predetermined value.
Also, the present invention provides a method of wire electric discharge machining by which an electric discharge is generated in a gap between a wire electrode and a workpiece so that the workpiece is machined, comprising the steps of: a first step in which electric discharge machining is conducted in a dielectric fluid; a second step in which electric discharge machining is conducted in mist; and a third step in which electric discharge machining is conducted in gas.
Also, the present invention provides a method of wire electric discharge machining, in which the machining process is changed over between said machining processes when straightness of the workpiece reaches a predetermined value.